Essentially water-free polymerized crystalline colloidal array composites having tunable radiation diffracting properties and process for making

ABSTRACT

The present invention is directed to a composite having tunable radiation diffracting properties which includes a flexible, water-free polymeric matrix and a crystalline colloidal array of particles having a lattice spacing, the array being embedded in the polymeric matrix and the lattice spacing changing responsive to stress applied to the polymeric matrix, thereby causing the radiation diffracting properties to change, wherein the polymeric matrix, the lattice spacing and the radiation diffracting properties all return to their original state essentially immediately upon removal of the stress. The present inventive composite is preferably made by a process, which involves forming a preliminary hydrogel polymerized crystalline colloidal array (PCCA), dehydrating the PCCA, and then forming a final, encapsulating polymeric matrix.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Priority is hereby claimed to Provisional U.S. Application SerialNo. 60/327,074, which is entitled: “POLYMERIZED CRYSTALLINE COLLOIDALARRAY COMPOSITES HAVING TUNABLE RADIATION DIFFRACTION PROPERTIES” filedOct. 3, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates generally to radiation filtersbased on crystalline colloidal arrays. More specifically, the presentinvention is directed to a tunable radiation filter, which includes ahighly ordered crystalline array of particles fixed in an essentiallywater-free matrix and a process for making such.

BACKGROUND OF THE INVENTION

[0003] A crystalline colloidal array (CCA) is a three dimensionallyordered lattice of self-assembled monodisperse colloidal particles,typically amorphous silica or a polymer latex, dispersed in an aqueousor non-aqueous medium. At high particle concentrations, long-rangeelectrostatic interactions between particles result in a significantinter-particle repulsion, which yields the adoption of a minimum energycolloidal crystal structure with either body-centered cubic orface-centered cubic symmetry.

[0004] Crystalline colloidal arrays can be formed having latticespacings comparable to the wavelengths of ultraviolet, visible andinfrared radiation. It has long been recognized that an array comparablein period to the wavelength of electromagnetic waves can provide ananalog, i.e., a “bandgap,” which can act as a filter for a particularwavelength. Bragg diffraction techniques have been used to examine CCAswith a view towards identifying their interparticle spacing, latticeparameters and phase transitions. Because CCAs can be fabricated todiffract electromagnetic radiation, including the visible spectrum, sucharrays have potential applications as optical filters, switches,limiters and sensors. However, the low elastic modulus exhibited by aliquid dispersion results in weak shear, gravitational, electric field,or thermal forces having the propensity to disturb the crystalline orderand is a severe drawback to the practical application of CCAs inphotonic devices.

[0005] Recently, approaches to develop robust network matrices have beenpioneered to stabilize both organic and inorganic arrays through an insitu polymerization of a monomer around the ordered arrays.Specifically, colloidal crystals arrays have been stabilized throughencapsulation in hydrogel networks and have been referred to aspolymerized crystalline colloidal arrays (PCCAs). However, the PCCAscontain at least 30 percent by volume of water, resulting in theirfragility and propensity for significant changes in optical performancewith water content.

[0006] To overcome the drawbacks of the hydrogel networks CCAs have beenencapsulated in essentially water-free polymeric matrices. However, onemotivation for developing a more robust system was to achieve varyingtypes of tunability, i.e., controllable changes of the CCA latticespacings responsive to specific environmental stimuli. Yet, thewater-free PCCAs that have been formed to date have exhibited limitedtuning capabilities. Specifically, prior art composite films composed ofsilica particles in an acrylate polymeric matrix have exhibited bandstop tuning responsive to mechanical stress, though the diffractionwavelength shifts were limited to about 50 nm or less and the time forthe films to return to the optical characteristics of their unloadedstate after the cessation of stress was from two to four hours.

[0007] Accordingly, there exists a need in the art for robust compositeswhich exhibit radiation diffracting properties, which are tunable to asignificant degree responsive to applied stress and which return totheir initial optical characteristics immediately upon the cessation ofstress.

SUMMARY OF THE INVENTION

[0008] Accordingly, the present invention is directed to a compositehaving tunable electromagnetic wave diffracting properties, whichincludes a flexible, essentially water-free polymeric matrix and acrystalline colloidal array of particles having an initial latticespacing, the array embedded in the polymeric matrix, wherein the latticespacing changes responsive to environmental stimulation applied to thepolymeric matrix, thereby causing the electromagnetic wave diffractingproperties to change, and wherein the array reverts to the initiallattice spacing essentially immediately upon removal of theenvironmental stimulation. Preferably, the particles of the crystallinecolloidal array are charged particles. Preferably the particles of thecrystalline colloidal array are polymeric. Alternatively, the particlesof the crystalline colloidal array may be inorganic. Preferably, theessentially water-free matrix is an acrylate polymer. It is alsopreferred that the essentially water-free matrix is elastomeric. It isalso preferred that the essentially water-free matrix is crosslinked.The electromagnetic wave diffracting properties of the composite willchange responsive to environmental stimulation including mechanicalstimulation, thermal stimulation, electrical stimulation, and chemicalstimulation.

[0009] The present invention is also directed to a method for making acomposite having tunable electromagnetic wave diffracting properties,which includes the steps of:

[0010] a) allowing colloidal particles to self-assemble into acrystalline colloidal array in a medium;

[0011] b) adding at least one hydrogel-forming monomer to the mediumcontaining the crystalline colloidal array;

[0012] c) polymerizing the at least one hydrogel-forming monomer to forma polymerized crystalline colloidal array having a hydrogel matrix;

[0013] d) dehydrating the hydrogel matrix of the polymerized crystallinecolloidal array;

[0014] e) swelling the dehydrated polymerized crystalline colloidalarray with at least one monomer; and

[0015] f) polymerizing the at least one monomer thereby forming anessentially water-free polymerized crystalline colloidal array.

[0016] Preferably, the colloidal particles are charged. Preferably, thehydrogel matrix is a polyethylene glycol. Alternatively, the hydrogelmatrix may be a polyacrylamide. The at least one monomer employed inswelling the dehydrated crystalline colloidal array may be a liquidmonomer or a solid monomer dissolved in a solvent.

[0017] The present invention is also directed to a photonic compositehaving a tunable bandgap which is made by a process including the stepsof:

[0018] a) allowing colloidal particles to self-assemble into acrystalline colloidal array in a medium;

[0019] b) adding at least one hydrogel-forming monomer to the mediumcontaining the crystalline colloidal array;

[0020] c) polymerizing the at least one hydrogel-forming monomer to forma polymerized crystalline colloidal array having a hydrogel matrix;

[0021] d) dehydrating the hydrogel matrix of the polymerized crystallinecolloidal array;

[0022] e) swelling the dehydrated polymerized crystalline colloidalarray with at least one monomer; and

[0023] f) polymerizing the at least one monomer thereby forming anessentially water-free polymerized crystalline colloidal array;

[0024] wherein the bandgap shifts responsive to environmentalstimulation.

[0025] Preferably the hydrogel matrix is a polyethylene glycol.Alternatively, the hydrogel matrix may be a polyacrylamide. Preferably,the at least one monomer employed in swelling the dehydrated polymerizedcrystalline colloidal array is an acrylate monomer.

[0026] Thus, the present invention is directed to a composite havingtunable radiation diffracting properties which includes a flexible,water-free polymeric matrix and a crystalline colloidal array ofparticles having a lattice spacing, the array being embedded in thepolymeric matrix and the lattice spacing changing responsive to stressapplied to the polymeric matrix, thereby causing the radiationdiffracting properties to change, wherein the polymeric matrix, thelattice spacing and the radiation diffracting properties all return totheir original state essentially immediately upon removal of the stress.

[0027] Preferably, the polymeric matrix is elastomeric. One preferredpolymer for use as the present polymeric matrix is poly(2-methoxyethylacrylate) although any of a nearly limitless number of polymers havingappropriate optical and mechanical properties may be employed.

[0028] Polystyrene particles are preferred for the crystalline colloidalarray of the present invention although, hereagain, any suitableparticles can be used. Examples of such include polymethylmethacrylate,silicon dioxide, aluminum oxide, polytetrafluoroethylene or any othersuitable materials which are generally uniform in size and surfacecharge.

[0029] The CCA spacing may provide for radiation diffracting propertiesin the ultraviolet, visible and/or infrared portion or portions of theelectromagnetic spectrum. Preferably, changes in the lattice spacing ofthe crystalline colloidal array effect changes in the compositeradiation diffracting properties which result in diffraction wavelengthshifts of as much as 55 nm or more, more preferably, as 100 nm or more.The recovery time for returning to the initial lattice spacingconfiguration upon removal of stress, as is evidenced by a return to adiffraction wavelength within 2 nm of the original diffractionwavelength, is less than one minute, preferably less than 10 seconds andmost preferably less than two seconds.

[0030] Further, the present invention is directed to a method for makinga composite having tunable radiation diffracting properties whichincludes the steps of:

[0031] a) allowing colloidal particles to self-assemble into acrystalline colloidal array in a medium;

[0032] b) adding at least one hydrogel-forming monomer to the mediumcontaining the crystalline colloidal array;

[0033] c) polymerizing the at least one hydrogel-forming monomer to forma polymerized crystalline colloidal array having a hydrogel matrix;

[0034] d) dehydrating the hydrogel matrix of the polymerized crystallinecolloidal array;

[0035] e) swelling the dehydrated polymerized crystalline colloidalarray by adding at least one further monomer having an affinity for thehydrogel; and

[0036] f) polymerizing the at least one further monomer thereby formingan essentially water-free polymerized crystalline colloidal array.

[0037] One preferred monomer for use as the hydrogel-forming monomer isethylene glycol. Alternatively, acrylamide may be employed. A preferredmonomer for use as the at least one further monomer for forming anessentially water-free polymerized crystalline colloidal array is2-methoxyethyl acrylate although any of a wide variety of monomerscapable of forming essentially water-free polymers having appropriateoptical and mechanical properties may be employed.

[0038] Polystyrene particles are preferred for forming theself-assembled crystalline colloidal array of the present inventionalthough any suitable particles can be used. Examples of such includepolymethylmethacrylate, silicon dioxide, aluminum oxide,polytetrafluoroethylene or any other suitable materials which aregenerally uniform in size and surface charge.

[0039] Additionally, the present invention is directed to a compositehaving tunable radiation diffracting properties made by a process whichincludes the steps of:

[0040] a) allowing colloidal particles to self-assemble into acrystalline colloidal array in a medium;

[0041] b) adding at least one hydrogel-forming monomer to the mediumcontaining the crystalline colloidal array;

[0042] c) polymerizing the at least one hydrogel-forming monomer to forma polymerized crystalline colloidal array having a hydrogel matrix;

[0043] d) dehydrating the hydrogel matrix of the polymerized crystallinecolloidal array;

[0044] e) swelling the dehydrated polymerized crystalline colloidalarray by adding at least one further monomer having an affinity for thehydrogel; and

[0045] f) polymerizing the at least one further monomer thereby formingan essentially water-free polymerized crystalline colloidal array;

[0046] wherein the essentially water-free polymerized crystallinecolloidal array has a lattice spacing which changes responsive to stressthereby changing the radiation diffracting properties and wherein theessentially water-free polymerized crystalline colloidal array, thelattice spacing and the radiation diffracting properties return to theiroriginal states essentially immediately upon release of the stress.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] A full and enabling disclosure of this invention, including thebest mode shown to one of ordinary skill in the art, is set forth inthis specification. The following Figures illustrate the invention:

[0048]FIG. 1 is a schematic representation of the process of the presentinvention; and

[0049]FIG. 2 is reflectance spectra of a composite in accordance withthe present invention prior to, during, and following the application of145 kPa compressive stress.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0050] It is to be understood by one of ordinary skill in the art thatthe present discussion is a description of exemplary embodiments only,and is not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

[0051] The present invention is directed to a composite having tunableradiation diffracting properties, i.e., a tunable photonic bandgap. Inessence a crystalline colloidal array (CCA) of particles is “frozen” inits optimum configuration and embedded in a flexible, water-freepolymeric matrix thus forming a “photonic crystal” which can forbidphoton transport for a certain band of frequencies. The lattice spacingof the CCA changes responsive to stress applied to the polymeric matrix,thereby causing the radiation diffracting properties to change. Thus, byapplying a predetermined amount of stress to the polymeric matrix thecomposite can be tuned from one set of optical properties to another. Asis noted above, similar composites have been made in the past.

[0052] However, the present composite exhibits improved opticalproperties, greatly improved tunability, and most importantly, theability to recover both mechanically and optically almost immediatelyupon the removal of stress. These improvements are achieved by a processfor forming the present composite which first locks in the original,optimum configuration of the CCA prior to forming the final polymericmatrix about the colloidal particles.

[0053]FIG. 1 schematically illustrates the present method for forming atunable composite in accordance with the present invention. Mostbasically, colloidal particles 10 are allowed to self-assemble into acrystalline colloidal array 12 within a medium 14. In order to initiallystabilize the array, a hydrogel-forming monomer is added to the medium14 and polymerized, as by photoinitiated free radical polymerization,into a hydrogel 16, thereby forming a hydrogel-based polymerizedcrystalline colloidal array (PCCA) 18. Thereafter, the water is removedby evaporation to form dehydrated hydrogel-based PCCA 20. The dehydratedPCCA is then swollen in a liquid monomer which has a strong affinity forthe hydrogel-based matrix. That monomer is then polymerized, preferablyphotopolymerized, into an essentially water-free polymeric matrix 22thereby forming final composite 24.

[0054] The colloidal particles 10 can be colloidal polystyrene,polymethylmethacrylate, polybutadiene, polyisoprene, silicon dioxide,aluminum oxide, polytetrafluoroethylene or any other suitable materialswhich are generally uniform in size and surface charge. Colloidalpolystyrene is preferred. The particles are chosen depending upon theoptimum degree of ordering and the resulting lattice spacing desired forthe particular application.

[0055] In general, any emulsion system including macro ions could beemployed to form the CCA. In one embodiment, a CCA can be formed of ahybrid of two or more different particle types. For example, a CCA canbe formed of a hybrid of two different types of organic particles orinorganic particles. Alternatively, a CCA can be formed of a hybrid ofboth inorganic and organic particles. In general, the macro ionsutilized can be spherical in shape, though this is not a requirement.

[0056] A CCA will usually fall into one of two general categories. Forexample, a CCA can be a sterically packed array, in which the colloidalparticles can usually be from about 10 nm to about 1 μm in diameter.Specifically, the particles can be from about 50 to about 800 nm indiameter. More specifically, the particles can be from about 200 toabout 500 nm in diameter. Whatever the chosen diameter of the particles,the particles can contact each other to form an ordered, packed system.

[0057] Alternatively, a CCA can be an electrostatically stabilizedsystem, in which the colloidal particles are produced such that theyexhibit a negative surface charge. When placed in a solution which ispure and nearly free of ionic species, the repulsive interaction betweenthe macro ions can be significant over distances greater than 1 μm. Whendispersed in a polar medium at high particle concentrations,interactions between the surface charge of the particles, coupled withthe consequent diffuse counterion cloud (known as a double layer effect)can result in the adoption of a minimum energy crystalline colloidalstructure having either a body centered cubic (bcc) or face centeredcubic (fcc) symmetry.

[0058] As noted above, these CCA systems can be fabricated to exhibitspecific periodicity analogous to electromagnetic wavelengths. Forexample, the periodicity of the array can be analogous toelectromagnetic wavelengths in the infrared, visible, or ultravioletspectrums. This can result in the appearance of a bandgap in thespectrum. The refractive index of these systems can be further adjustedthrough the addition of various additives. For example, dyes,photochromic dyes, or fluorine can be incorporated into the CCA to“tune” the optical effects of the system. CCAs exhibiting opticalbandgap effects can then be employed in a variety of active photonicswitching and sensory roles.

[0059] The composites of the present invention have preferably beenformed from electrostatically stabilized CCA systems, althoughsterically packed CCAs may alternatively be employed. In one embodiment,the CCAs utilized can be formed using monodisperse cross-linkedpolystyrene-based particles as the colloidal particles, though this isnot required for practice of the invention. These particles can beprepared by using standard emulsion polymerization procedures, which areknown in the art.

[0060] Specifically, an emulsion polymer colloid can be prepared bymixing the desired monomer with a cross-linking agent, a surfactant toaid in the formation of the emulsion, a buffer to keep the pH of thesolution constant and to prevent particle coagulation, and afree-radical initiator to initiate polymerization. In one embodiment themonomer is styrene, the cross-linking agent is divinylbenzene, thesurfactant is sodium lauryl sulfate, the initiator is potassiumpersulfate and, optionally, an ionic comonomer is also added, such as1-sodium, 1-allyloxy-2-hydroxypropane sulfonate. Other suitablecompounds can also be used to prepare the emulsion polymer colloid, solong as compatibility problems do not arise. The particles should thenbe purified by the use of centrifugation, dialysis and/or an ionexchange resin. Purification of the commercially available particles isalso required.

[0061] Following polymerization, the particles may be stored in an ionexchange resin. The ion exchange resin should preferably be cleanedprior to use.

[0062] The colloidal particles employed can be of any suitable particlesize, but in general will be between about 10 nanometers to about 10microns in diameter. Specifically, the particles can be between about 20and about 500 nanometers in diameter. More specifically, the particlescan be between about 100 and about 200 nanometers in diameter.

[0063] In one possible embodiment, the resulting latex produced by theemulsion polymerization procedures can be dialyzed against deionizedwater and then shaken with an excess of mixed bed ion-exchange resin toremove excess electrolyte. The CCA can then be allowed to self-assemble.

[0064] The electrically charged particles are then allowed to selfassemble to form a crystalline colloidal array. This assembly takesplace in a suitable medium 14, preferably water.

[0065] The diffraction characteristics of CCA systems are mostaccurately predicted through the application of dynamic diffractiontheory, though Bragg's law is a reasonable approximation. Of importanceto the present invention, a CCA can be “tuned” to exhibit some desiredperiodicity and exhibit a specific bandgap based on the interplanerspacing of the diffracting lattice planes. Interplaner spacing in turncan be a function of the concentration of colloidal particles formingthe CCA. In other words, the concentration of colloidal particles can bedesigned or altered in order that the CCA exhibit a specific bandgap.

[0066] Conversely, a shift in the observed bandgap of the system can beevidence of a shift in the interplaner spacing, d, of the orderedsystem. Such a shift in the ordered lattice structure may beattributable to some specific stimulation of the system. For example,when a CCA is formed in a deionized water system, the CCA can opalesceat a certain color due to the optical bandgap effect. If water or someother compound in the system is allowed to escape, due to, for example,evaporation, the observed bandgap can shift due to the increasedconcentration of the colloidal particles and the decreased interplanerspacing of the array. As such, the system will opalesce at a bluer huedue to the change in particle concentration.

[0067] Similarly, the addition of a compound to the system can cause adecrease in the concentration of colloidal particles and a relativeincrease in the interplaner spacing of the array, thus a red shift inthe optical bandgap can be seen. As a result, such CCAs can be useful invarious optical switching and sensing technologies.

[0068] Following formation of the CCA, the hydrogel monomer is added andpolymerized to form an encapsulated hydrogel polymerized crystallinecolloidal array (PCCA). The PCCA can be formed by any suitable method.In general, such methods can be thin film formation methods. This couldinclude lithography methods, such as, for example, photolithography,various forms of near-field optical lithography, and soft lithography.Alternatively, other forms of thin film formation could be utilized suchas surface templating, layer-by-layer assembly methods, pulsed laserdeposition methods, or through polymerization of the CCA/monomer blendsolution within a defined area. For example, in one embodiment, theCCA/monomer solution can be an aqueous solution including aphotoinitiator injected between two quartz plates separated by aParafilm spacer and then polymerized into a PCCA hydrogel throughexposure to an ultraviolet electromagnetic radiation source for asuitable period of time. In general, no matter what method of productionis used, the product PCCA can have a size defined by the desired finalapplication of the film. For example, the PCCA film can be from about 1to about 1500 μm thick and have length and width dimensions as required.

[0069] To provide for more efficient polymerization of the monomer, apolymerization initiator can be added to the CCA/monomer blend. Forexample, in one embodiment, the polymerization process can be aphotopolymerization process. Photopolymerization, though not required,has proven effective due to the limitation of possible disturbing forceswhich could disrupt the ordered system. In this particular embodiment, aphotoinitiator can be added to the CCA/monomer blend. Any suitablephotoinitiator can be used such as, for example, benzoin methyl ether(BME) or 2,2′-diethoxyacetophenone (DEAP). Usually, only a small amountof a photoinitiator is necessary for polymerization of the monomer tooccur. For example, ratios of photoinitiator to monomer can be fromabout 1:100 to about 5:100 to effect polymerization as desired.

[0070] Upon polymerization, the hydrogel polymer can form either athermoplastic or a thermoset network, as desired, around the orderedcolloidal particles. If a thermoset polymerized system is desired, acrosslinking agent may be added to the CCA/monomer blended system priorto polymerization. In general, a crosslinking agent can be added to themonomer in a ratio of from about 1:5 to about 1:20 (crosslinking agentto monomer). More specifically, the ratio of crosslinking agent tomonomer can be from about 1:8 to about 1:15.

[0071] Any suitable crosslinking agent can be utilized. In general, inan embodiment involving a polyethylene glycol (PEG)-based hydrogel PCCA,the crosslinking agent can also be a PEG-based agent, though this is nota requirement. A non-exhaustive list of possible crosslinking agents caninclude, but is not limited to: poly(ethylene glycol) dimethacrylate,poly(ethylene glycol) diacrylate, poly(ethylene glycol) divinyl ether,poly(ethylene glycol) dioleate, and N,N′ methylene bis acrylamide.

[0072] Thus, preferably, a hydrogel-forming monomer is then added to thecrystalline colloidal array medium, along with a cross-linker and aphotoinitiator. Monomers of poly(acrylamide) and its derivatives arewell known as hydrogel-formers and may be employed in accordance withthe present invention. However, poly(ethylene glycol) (PEG) hydrogelsare preferred in accordance with the present invention as it has beendetermined that networks based on PEG may provide more versatile tuningproperties. Of course, a variety of other hydrogel homopolymers andcopolymers may also be advantageously employed in accordance with thepresent invention depending on compatibility with the subsequent, finalmatrix chemistry. In the preferred embodiment, however, thehydrogel-forming monomer is a monomer of poly(ethylene glycol)methacrylate, the cross-linker is poly(ethylene glycol) dimethacrylate,and the photoinitiator is 2,2-diethoxyacetophenone. Photopolymerization,accomplished by exposure to a UV source in the presence of aphotoinitiator, is the preferred means for forming the present hydrogel.

[0073] Following its formation, the hydrogel matrix containing theembedded crystalline colloidal array is completely dehydrated to formdehydrated polymerized crystalline colloidal array 20. Preferably, thePCCA is allowed to air dry for a period of days and then placed in avacuum oven in order to ensure complete dehydration.

[0074] Following dehydration, the dehydrated PCCA is then swollen with aliquid monomer or monomers that will eventually form the matrix 22 offinal composite 24. Any of a wide variety of polymers may be employedfor matrix 22 depending on the properties desired for the overallcomposite. In fact, it has been determined in accordance with thepresent invention that at least the thermal and mechanical properties ofthe final composite are determined almost exclusively by the finalmatrix polymer employed and are not affected significantly by thecompositions of the colloidal particles or the hydrogel. However,certain considerations must be taken into account in choosing anappropriate matrix polymer.

[0075] Primarily, the monomer or monomers which will eventually form thefinal matrix polymer must have an affinity for the hydrogel. Basically,this affinity may be viewed as an indication of mutual solubility suchthat a monomer which has an affinity for a given hydrogel will readilyswell the dehydrated hydrogel PCCA like water swelling a dried sponge. Amonomer which does not have an affinity for the particular hydrogelwhich has been employed will be repelled by the dehydrated hydrogel PCCAand will not swell it. Of course, this required affinity can beanticipated and employed as a determining factor in choosing anappropriate hydrogel which is compatible to a desired final matrixpolymer. Thus, for a PEG hydrogel, a preferred monomer is a PEGfunctionalized acrylate such as 2-methoxyethyl acrylate.

[0076] Also of primary importance is that the monomer or monomersemployed must be a precursor or precursors to an essentially water-freematrix. That is, when polymerized the monomer or monomers must form ahomopolymer or copolymer which is not a hydrogel. The formation of anessentially water-free, non-hydrogel, polymeric matrix is key toachieving the robust, readily tunable composite of the presentinvention.

[0077] Secondly, it is preferred that the monomer which swells thedehydrated hydrogel PCCA and eventually forms the final matrix polymeris a liquid. However, a solid monomer dissolved in an appropriatesolvent may also be employed.

[0078] Finally, depending on the particular end-use application, it maybe desirable for the final composite to possess certain optical, thermaland/or mechanical properties. As was noted above, these may all betailored by choosing an appropriate matrix polymer. For example,although a preferred monomer for forming the final matrix polymer is2-methoxyethyl acrylate, it has been found in accordance with thepresent invention that the glass transition temperature of the finalcomposite can be altered by copolymerization of additional acrylatemonomers or through a complete substitution. For example, the glasstransition temperature of the composite made in accordance with Example2, below, is about −35° C. The glass transition temperature of thecomposite made in accordance with Example 3, below, is in the range of40-50° C. The composite made in accordance with Example 4, below, has acopolymer matrix wherein the dehydrated hydrogel PCCA has been swollenwith a 50/50 blend of 2-methoxyethyl acrylate and 2-methoxyethylmethacrylate, the two monomers employed in Examples 2 and 3,respectively. Thus, the composite of Example 4 has a glass transitiontemperature between those other two composites. Thus, in one preferredembodiment, 2-methoxyethyl acrylate including 1% by weight of ethyleneglycol dimethacrylate is employed to swell the dehydrated PEG-basedhydrogel PCCA. A photoinitiator such as 2,2-diethoxyacetophenone isadded. Subsequent photopolymerization yields a water-free, robustcomposite.

[0079] Preferably, the polymer chosen for use as the final compositematrix material is sufficiently flexible, most preferably elastomeric,in order to provide a final composite which is capable of exhibitingmechanochromic properties. That is, deformation of the matrix by appliedstress causes deformation of the embedded crystalline colloidal arraylattice spacing and, therefore, a change in the radiation diffractingproperties of the composite. However, it should be noted that presentinventive composite may be tailored to exhibit tunability responsive toother environmental stimuli such as, for example, changes intemperature, exposure to certain chemicals, or exposure to electric orelectromagnetic fields.

[0080] A particular advantage of the present invention, however, is theability to provide PCCA composites which are tunable responsive toapplied stress and, most particularly, which are capable of returning tothe original unstressed state, both physically and optically, withinseconds of removal of the applied stress. That is, upon removal ofapplied stress composites in accordance with the present inventionreturn to a diffracted bandwidth within 1 to 2 nanometers of theoriginal unstressed bandwidth essentially immediately.

[0081] Reference now will be made to possible embodiments of theinvention, one or more examples of which are set forth below. Eachexample is provided by way of explanation of the invention, not as alimitation of the invention. In fact, it will be apparent to thoseskilled in the art that various modifications and variations can be madein this invention without departing from the scope or spirit of theinvention. For instance, features illustrated or described as part ofone embodiment can be used on another embodiment to yield a stillfurther embodiment. Thus, it is intended that the present inventioncover such modifications and variations as come within the scope of theappended claims and their equivalents.

EXAMPLE 1

[0082] Monodisperse crosslinked polystyrene particles were preparedusing an emulsion polymerization procedure which involved mixing styrenemonomer with divinylbenzene, a cross-linking agent, sodium laurylsulfate, a surfactant, and potassium persulfate, a free-radicalinitiator for initiating polymerization. The resulting particles weredialyzed against deionized water and then shaken with an excess of mixedbed ion-exchange resin. After cleaning, the particle diameter wasmeasured to be 109±26 nm. The cleaned suspension with then diluted withdeionized water until an angle dependent iridescence was observed.Drying a known mass of the suspension in an oven at 80° C. overnightthen under vacuum for two days resulted in a calculated particle densityof 2.6×10¹⁴ cm⁻³.

[0083] The crystalline colloidal arrays which formed in the deionizedwater were encapsulated in a hydrogel matrix prepared by an in situphotopolymerization procedure. The matrix materials included a monomerof poly(ethylene glycol) methacrylate (PEGMA, M_(n)=360), a crosslinkerof poly(ethylene glycol) dimethacrylate (PEGDMA, M_(n)=550), and aphotoinitiator of 2,2-diethoxyacetophenone (DEAP). The PEGMA and PEGDMAwere stored in Nalgene containers over a mixed bed ion-exchange resinfor at least 48 hours prior to their use, while all other matrixmaterials were used as-received.

[0084] The procedure for generating a hydrogel polymerized crystallinecolloidal array film included combining all the components of the PCCAin a Nalgene container and allowing the mixture to shake with an excessof a mixed bed ion-exchange resin for at least two hours prior toinjecting the mixture between two quartz plates separated by a 500 μmparafilm spacer. The film was then polymerized by exposing the assemblyto a UV source for four minutes.

EXAMPLE 2

[0085] A hydrogel PCCA film made in accordance with Example 1 wasremoved from the quartz plates and allowed to air dry for two days andthen placed in a vacuum oven at 35° C. The resulting clear film was thenswollen in a solution of 2-methoxyethyl acrylate for two days. To thissolution, ethylene glycol dimethacrylate and DEAP was added and theformulation was crosslinked by a twenty-minute exposure to a UV lamp.All chemicals were purchased from either Aldrich or Acros Organics.

[0086]FIG. 2 is a reflectance spectra of the composite of the presentExample collected on an Ocean Optics PC2000 fiber optic spectrometertaken at normal incidence in an initial stress-free state, under acompressive loading, and upon removal of the applied stress. In theinitial stress-free state, the position of the band stop is at 610 nm.Upon applying a 145 kPa compressive stress, the band stop shifts down toa wavelength of 517 nm, a 93 nm variation. Additional compressive stressresulted in increasingly larger band stop shifts, with shifts of 120 nmbeing attainable. However, with increasing stress, the peaks becamebroader and less well defined due to the introduction of disorder in thearray. As is shown in FIG. 2, removal of the compressive stress resultsin the immediate return of the band stop position within 1-2 nm of theoriginal stress-free state. It was found that repeated straining of acomposite in accordance with the present invention did not result in anypermanent change in the observed optical characteristics unless the filmwas mechanically degraded.

EXAMPLE 3

[0087] A hydrogel PCCA film made in accordance with Example 1 wasremoved from the quartz plates and allowed to air dry for two days andthen placed in a vacuum oven at 35° C. The resulting clear film was thenswollen in a solution of 2-methoxyethyl methacrylate for two days. Tothis solution, ethylene glycol dimethacrylate and DEAP was added and theformulation was crosslinked by a twenty-minute exposure to a UV lamp.All chemicals were purchased from either Aldrich or Acros Organics.

EXAMPLE 4

[0088] A hydrogel PCCA film made in accordance with Example 1 wasremoved from the quartz plates and allowed to air dry for two days andthen placed in a vacuum oven at 35° C. The resulting clear film was thenswollen in a 50/50 solution of 2-methoxyethyl acrylate and2-methoxyethyl methacrylate for two days. To this solution, ethyleneglycol dimethacrylate and DEAP was added and the formulation wascrosslinked by a twenty-minute exposure to a UV lamp. All chemicals werepurchased from either Aldrich or Acros Organics.

[0089] These and other modifications and variations to the presentinvention may be practiced by those of ordinary skill in the art,without departing from the spirit and scope of the present invention,which is more particularly set forth in the appended claims. Inaddition, it should be understood that aspects and various embodimentsmay be interchanged either in whole or in part. Furthermore, those ofordinary skill in the art will appreciate that the foregoing descriptionis by way of example only, and is not intended to limit the invention sofurther described in such appended claims.

What is claimed is:
 1. A composite having tunable electromagnetic wavediffracting properties, comprising: a flexible, essentially water-freepolymeric matrix; and a crystalline colloidal array of particles havingan initial lattice spacing, the array embedded in the polymeric matrix,wherein the lattice spacing changes responsive to environmentalstimulation applied to the polymeric matrix, thereby causing theelectromagnetic wave diffracting properties to change, and wherein thearray reverts to the initial lattice spacing essentially immediatelyupon removal of the environmental stimulation.
 2. The composite setforth in claim 1 wherein the particles of the crystalline colloidalarray comprise charged particles.
 3. The composite set forth in claim 1wherein the particles of the crystalline colloidal array comprisepolymeric particles.
 4. The composite set forth in claim 1 wherein theparticles of the crystalline colloidal array comprise inorganicparticles.
 5. The composite set forth in claim 1 wherein the essentiallywater-free matrix comprises an acrylate polymer.
 6. The composite setforth in claim 1 wherein the essentially water-free matrix iselastomeric.
 7. The composite set forth in claim 1 wherein theessentially water-free matrix is crosslinked.
 8. The composite set forthin claim 1 wherein the environmental stimulation comprises mechanicalstimulation.
 9. The composite set forth in claim 1 wherein theenvironmental stimulation comprises thermal stimulation.
 10. Thecomposite set forth in claim 1 wherein the environmental stimulationcomprises electrical stimulation.
 11. The composite set forth in claim 1wherein the environmental stimulation comprises chemical stimulation.11. A method for making a composite having tunable electromagnetic wavediffracting properties comprising the steps of: a) allowing colloidalparticles to self-assemble into a crystalline colloidal array in amedium; b) adding at least one hydrogel-forming monomer to the mediumcontaining the crystalline colloidal array; c) polymerizing the at leastone hydrogel-forming monomer to form a polymerized crystalline colloidalarray having a hydrogel matrix; d) dehydrating the hydrogel matrix ofthe polymerized crystalline colloidal array; e) swelling the dehydratedpolymerized crystalline colloidal array with at least one monomer; andf) polymerizing the at least one monomer thereby forming an essentiallywater-free polymerized crystalline colloidal array.
 12. The method setforth in claim 11 wherein said colloidal particles are charged.
 13. Themethod set forth in claim 11 wherein the hydrogel matrix comprises apolyethylene glycol.
 14. The method set forth in claim 11 wherein thehydrogel matrix comprises a polyacrylamide.
 15. The method set forth inclaim 11 wherein the step of swelling the dehydrated polymerizedcrystalline colloidal array with at least one monomer comprises swellingthe dehydrated polymerized crystalline colloidal array with a liquidmonomer.
 16. The method set forth in claim 11 wherein the step ofswelling the dehydrated polymerized crystalline colloidal array with atleast one monomer comprises swelling the dehydrated polymerizedcrystalline colloidal array with a solid monomer dissolved in a solvent.17. A photonic composite having a tunable bandgap made by a processcomprising the steps of: a) allowing colloidal particles toself-assemble into a crystalline colloidal array in a medium; b) addingat least one hydrogel-forming monomer to the medium containing thecrystalline colloidal array; c) polymerizing the at least onehydrogel-forming monomer to form a polymerized crystalline colloidalarray having a hydrogel matrix; d) dehydrating the hydrogel matrix ofthe polymerized crystalline colloidal array; e) swelling the dehydratedpolymerized crystalline colloidal array with at least one monomer; andf) polymerizing the at least one monomer thereby forming an essentiallywater-free polymerized crystalline colloidal array; wherein the bandgapshifts responsive to environmental stimulation.
 18. The photoniccomposite set forth in claim 17 wherein the hydrogel matrix comprises apolyethylene glycol.
 19. The photonic composite set forth in claim 17wherein the hydrogel matrix comprises a polyacrylamide.
 20. The photoniccomposite set forth in claim 17 wherein the at least one monomeremployed in swelling the dehydrated polymerized crystalline colloidalarray comprises an acrylate monomer.